The seats on my advance are quite dished ,ie bucket seats. Very similar to the Ex but I dont know if its different on other trim levels.
Total width at widest ,including contoured side cushioning 50cm ,
Width of the flat lower bucket panel where your bottom rests, is 30 cm widening to 40 cm towards the front where your thighs are . The remaining width is tapered raised cushions that help to keep you in place when cornering. The centre section up the backrest is also 30cm plus contoured side cushioning.
On my first long drive in my previous brand new EX I did find it a bit uncomfortable along the back of my thighs compared to my old ,well used , Yaris. But soon found them comfortable even on long journeys .
I have no particular back problems and cant compare with a mk3 as I have never sat in one.
The partial proof of the pudding is actually sitting in a mk4 .